The present invention is directed to fluid-driven pumps. It finds particular, although not exclusive, use in fluid-driven pumps that are employed in interposing an unpressurized reservoir in a pressurized fluid line.
U.S. Pat. No. 4,658,760 to Zebuhr describes an improvement in water heaters and other devices that must interpose a reservoir in a pressurized fluid line in order to perform some operation, such as the heating of the water. In a conventional water heater, the water is heated at line pressure, so the reservoir must take the form of a pressure vessel; the vessel must be strong enough to withstand the water pressure. In the arrangement disclosed in the Zebuhr patent mentioned above, however, the water is heated at ambient pressure so that the vessel that serves as the reservoir has to be strong enough to withstand only the pressure that results from the weight of the water. The pressure vessel can thus be much less expensive.
In order to provide a low-pressure reservoir and yet allow the pressure to remain high both upstream and downstream of the reservoir, the Zebuhr arrangement employs a fluid-driven pump. The pressure difference between the upstream section of the line and the low-pressure reservoir drives a pump that draws fluid back out of the reservoir and drives it into the downstream section of the line at high pressure. Although the Zebuhr arrangement holds out the prospect of a significant reduction in the cost of water heaters and similar devices, acceptance of water heaters that follow the teachings of the Zebuhr patent will likely depend on the ability of such water heaters to meet certain criteria.
The first criterion is that pressure reduction be minimal. The fluid-driven pump must be arranged so that the flow from the upstream line results in at least an equal flow out of the pump. Otherwise, water will accumulate in the reservoir and cause it to overflow. (Of course, the overall system must result in the same volume rate of flow in the downstream line as in the upstream line, but a slight increase in flow rate through the pump is acceptable, since arrangements can be made to return excessive flow back into the reservoir.) Since the flow rate out must at least equal the flow rate in, the law of conservation of energy dictates that the theoretical upper limit on the pressure out be the pressure in. Any inefficiencies or flow-rate gains in the pump thus result in a pressure drop. Naturally, if such pressure drops are too great, they will reduce the desirability of water heaters of this type and in fact could eliminate their feasibility in some situations.
Another second acceptance criterion is that such heaters not introduce excessive noise into the system. The pump described in the Zebuhr patent is a reciprocating device, in which valves open and close at the end of each stroke. Such openings and closings necessarily result in the generation of pressure waves that propagate through the fluid lines. Although normal opening and closing of faucets also produces pressure waves, which are thus a normal and accepted in the water lines, the pressure waves could result in annoyance if their amplitude were great enough to result in significant noise.
A third acceptance criterion is that the pump not introduce significant pressure variations; such variations could cause shower water to flow alternately hot and cold.
It is accordingly an object of the present invention to minimize pressure variations that such fluid-driven pumps introduce.
It is another object of the present invention to minimize shock and noise introduced by pumps of this sort and to maximize their efficiency so that excessive pressure losses are avoided.